Soil Stability Hazards

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GEOTECHNICAL HAZARDS

GEOTECHNICAL

HAZARDS

APPROVING OFFICER ROLE

• APEGBC – Site safe for intended use?

• Rezoning

• Development Permit

• Subdivision

APPROVING OFFICER ROLE

• The Approving Officer may reject an application if the AO considers that:

• the land is subject, or could reasonably be expected to be subject, to erosion,

land slip or avalanche. This is obviously of particular significance on hillsides.

Some evidence that the Approving Officer may look for during a site inspection

to determine if a hazard exists is:

• debris at the bottom of a slope;

• evidence of rock fall;

• pistol butt trees (i.e., trees that grow out from the side of a steep slope at a near

horizontal angle and then curve up to take the shape of a pistol - these trees are

indicative of steep and potentially unstable banks); or

• cracks or fissures in a slope.

WHEN IN DOUBT ASK THE EXPERTS

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GEOTECNICAL

HAZARDS

Introduction

Current Legislation

Seismic Considerations

Settlement

Slope Stability

Stability Assessments

Stabilization Methods

Global Stability Considerations

Approval Officer Role

Question and Answer


Hazard – Source of potential harm, in terms of

human injury, damage to health or property.

GEOTECHNICAL

HAZARDS

What’s New?

- Recent changes to the Building Code

- Legislated Landslide Assessments

- Letters of Assurance

- Schedule B1, B2, and CB


Hazard – Source of potential harm, in terms of

human injury, damage to health or property.

Geotechnical Hazards:

• Seismic

• Settlement

• Slopes

• Flooding

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Seismic hazards that require geotechnical

consideration:

• Liquefaction

• Slope Instability


Seismic Liquefaction:

Liquefaction

Liquefaction Induced Lateral

Spread and Slope Instability

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SOIL STABILITY HAZARDS


Seismic Hazard Mitigation Technques

Vibro-Replacement Dynamic Compaction

Rapid Impact Compaction


Settlement


Settlement/Subsidence

Soils/areas susceptible to settlement:

• Peat and organic rich soils

• Soft silt and clay

• Uncontrolled fill

• Sites with underlying abandoned

utilities

• Abandoned mine workings

Settlement hazard mitigated by:

• Remove unsuitable soil

• Preload with a surcharge fill

• Injection of self levelling fill

material to fill voids

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Flatland/Valley Floor can also be impacted from

debris flows, debris floods and other landslide

runout.


Soil Slope Failure Modes


Rock Slope Failure Modes


Rock Slide - Abbotsford

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Circular Slump – Promontory


Tension Cracks – Impending Failure

Fraser Valley


Debris Avalanche - Old Orchard Road


Debris Avalanche - Chilliwack

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Stability Assessments:

Proactive Avoidance

Necessary Components

- Background Research

- Site Investigations by Qualified Professionals

- Terrain Assessments

- Stability Analyses

- Geotechnical Reports

Stability Assessments Stabilization Methods

Global Slope Stability

Levelton Consultants & the City of Chilliwack

Question and Answer


Surficial Geology

Pink: bedrock controlled

Light Green: Sumas drift / glaciolacustrine, glacial till / glaciolacustrine

Dark Green: windblown silt

Brown: mass wasting / landslides

Dark Orange: glaciofluvial (gravel and sand and marine sediments, clay)

Light Orange: glacial outwash, gravel and sand

Yellow: Fraser River sand and silt


Geotechnical Reports

Geotechnical reports need to be in accordance with:

-2012 Building Code

-2010 APEGBC Legislated Landslide Assessment Guidelines

Reports should address all relevant geotechnical hazards. The following provides an excellent

framework for assessing geohazards:

“HAZARD ACCEPTIBILITY THRESHOLDS FOR DEVELOPMENT APPROVALS BY LOCAL

GOVERNMENT“, (Revised Nov. 1993), Dr. Peter Cave.


Stabilization Methods:

When Slope Hazard Avoidance is

NOT Feasible

Implement Project-Specific

Stabilization Methods

Four Basic Stabilization Methods:

1. Improve Drainage

2. Decrease Load at Top of Slope

3. Buttress the Toe of Slope

4. Reinforce Slope Materials

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Roadside Drainage Improvements-

Promontory

Improve Drainage


Retaining Wall Buttress to Stabilize

Slope – Cherrywood Dr.

Decrease Load at Top of Slope

And Buttress the Toe


Buttressing Small Slump with RipRap

Buttress the Toe


Geogrid Reinforcement

Reinforce Slope Materials

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Soil Anchor Drilling /

Slope Reinforcement



Grouted Rock Bolts



Piled Retaining Wall With

“Deadmen” Tiebacks



Retaining Wall Stabilization - Ryder Lake Road

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Increased load on top of slope

reduces stability.

Example of REDUCING STABILITY


Excavation at the base of a slope

reduces stability.

Example of REDUCING STABILITY


Case Study

Rock slopes that appear

stable

may not be…


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Planar and Toppling Failure

(After)


Down-slope displacement from failure

on future building sites


Rock face stabilization with rock bolts,

horizontal drains, and shotcrete


Global Slope Stability

Individual projects affect

the global stability of site

and its surroundings.

Both site and surroundings

should be considered

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Municipalities should:

- Update Regional Overview Studies

- Require independent / 3rd party review of proposed steep slope sub-divisions, settlement sensitive

areas, and seismic hazard areas

- Initiate infrastructure risk assessments and review maintenance procedures


• QUESTIONS?

Thank You

Soil Stability Hazards

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